Plants prepare bituminous coated products for road dressings from recycled bituminous materials, virgin aggregates and liquid bitumen; these comprise a single drum in which the drying and heating of the solid materials, the incorporation of liquid bitumen in these materials and their mixing to obtain the bituminous coated products at the drum exit are carried out.
The drum, which comprises a cylindrical casing arranged with its lengthwise axis slightly inclined relative to the horizontal and mounted for rotation about this axis, may have passing through it a hot gas stream traveling between the material entry end and the drum exit end in the same direction as the solid materials. To permit recycled materials to be introduced, such a drum, generally called a parallel-stream drum, comprises a recycling ring arranged around the drum casing, which is provided with devices for introducing materials through its side wall, at the recycling ring. The recycling ring is situated in a region of the drum which is distant from its entry end through which cold virgin aggregates are introduced and through which a burner enters, ensuring the travel of hot gases in the drum, in the direction of travel of the solid materials.
Although the introduction of bituminous recycled materials and the incorporation of liquid bitumen in the solid materials, which is carried out downstream of the recycling ring, are carried out in a region which is relatively distant from the burner flame, inside the drum the materials containing bitumen are exposed to hot gases capable of producing some thermal degradation of these bituminous materials and the formation of blue smoke due to bitumen vaporization.
Also known are drums designed as counter current drums, which comprise a burner which has an elongate body entering through the exit end of the drum, so that the end of the burner body at which the flame is formed opens into a region of the drum which is distant from its exit end and from its entry end. In such a countercurrent drum, cold virgin aggregates can be introduced through the entry end of the drum, these cold aggregates then traveling in a first zone of the drum forming a drying and heating zone, countercurrentwise relatively to the travel of the hot gases originating from the zone situated inside the drum, in which the burner flame is developed. The liquid bitumen incorporated in the solid materials is introduced through the outlet end of the drum into a mixing zone situated around the burner body, downstream of the flame zone. The hot gases which travel in the drum between the flame zone and the entry end of the drum are therefore do not bass through the mixing zone.
Recycled bituminous materials can be introduced into the drum by a recycling ring situated level with the flame zone or slightly downstream of this zone.
In such a drum, the heat needed for the drying, heating and melting of the bitumen of the recycled materials is brought in solely by the virgin aggregates, which are mixed with the recycled materials before the incorporation of liquid bitumen.
Even in the case where operation of the drum results in considerable superheating of the virgin aggregates, it is not possible to incorporate a very large quantity of recycled, materials into these aggregates. The incorporation of recycled materials must be generally limited to a weight proportion of the order of 40% relative to the total weight flow of solid materials traveling in the drum. The incorporation of large quantities of recycled materials into the virgin aggregates becomes still more difficult in the case where the recycled materials are very wet, because a large quantity of energy must then be supplied to these recycled materials to vaporize the water which they contain. The incorporation of the recycled materials must then be limited to percentages well below 40% of the total weight flow of the solid materials.
To overcome these disadvantages it has been proposed to employ complex plants comprising a first drum drier with parallel streams, into which the cold and wet recycled materials are introduced, and a second, countercurrent drum, through the entry end of which the virgin aggregates are introduced.
The virgin aggregates and the dried and heated recycled materials recovered at the exit of the countercurrent drum, and of the parallel-stream drum respectively, are then introduced into a stationary mixing device in which the incorporation of the liquid bitumen and possibly of other additives is carried out. The gases for drying the recycled materials, which may contain a certain proportion of bitumen vapor, can be taken from the exit of the parallel-stream drum to be introduced into the countercurrent drum in the region of the burner flame.
Such devices are relatively complex and require the use of a stationary mixer such as a parallel-shaft mixer and of a set of means for storing and measuring out the solid materials before they are introduced into the mixer.